this catapult will shoot a 1/2 inch marble about 100 feet.
^this is a video of it shooting
^ this is a video of some of the dimensions on the catapult
Step 1: Frame
The base is a 2x2 foot square with two 3 foot uprights. The uprights are screwed into the inside of the box in the center of the frame.
Step 2: The Rest Of The Frame
the uprights have a piece of wood across the top that is 22 inches long. The throwing arm of the catapult is 3 and 1/2 feet long wit a bottle cap on the end of it. on the other end of the arm is the rod that is 15 inches from the bottom of the arm. the rod is in either upright at a height of 2 feet. there are 2 springs that I bought at lowes, and they are 12 inches long with a 1 inch diameter.
Step 3: Strengthen The Frame
the frame is fairly week if there is no extra strength and if there is nothing to compress the arm stress. there is one noodle that is cut up into 3 pieces to compress the stress. there is about 8-12 feet of wood just supporting the the 2x4 going across the uprights.